Electrode comprising a porous sintered body

ABSTRACT

A sintered electrode adapted to operate with an electric discharge arc appended to an outer surface thereof as one of the electrode components of high temperature discharge lamps such as gaseous and vapor discharge lamps and more particularly highpressure discharge lamps. The electrode comprises a central electrode rod having one portion substantially enclosed by two contiguous porous sintered zones each of which has an exposed outer surface. The exposed outer surface of the first of the two zones is positioned in the direction of the arc discharge during operation of the lamp and is closer proximity thereto during operation than the outer surface of the second zone. The first zone consists of a refractory material which has been sintered at a temperature higher than the temperature of the arc discharge to form a sintered interconnected porous material. The second zone consists of a sintered porous mixture of refractory material and emissive material.

United States Patent 1 91 1111 3,911,309

Kiimmel et al. 1 1 Oct. 7, 1975 [54] ELECTRODE COMPRISING A POROUS3,558,966 l/1971 Hill 313/346 R SINTERED BODY 3,656,020 4/1972 Cronin313/311 3,661,536 5/1972 Shimizu 313/311 [75] Inventors: Ulrich Kiimmel,Munich; Manfred Mair Augsburg both of Germany Primary Examiner-SaxfieldChatmon, Jr. [73] Assignee: Patent-Treuhand-Gesellschaft fiir Attorney,Agent, or FirmFlynn & Frishauf elektrische Gluhlampen mbH, Munich,Germany [57] ABSTRACT [22] Ffled: Sept 1973 A sintered electrode adaptedto operate with an elec- [21] Appl. No.: 396,446 tric discharge areappended to an outer surface thereof as one of the electrode componentsvof high temperature discharge lamps such as gaseous and [30] ForeignApphcatlon Pnomy Data vapor discharge lamps and more particularly high-Sept. 18, 1972 Germany 2245717 pressure discharge lamps The electrodeComprises a central electrode rod having one portion substantially [52]U.S. Cl 313/346 R; 29/25.]4; 29/25.]7; enclosed by two contiguous poroussimered Zones 313/311; 313/218 each of which has an exposed outersurface. The ex- [51 -Int. Cl. HOIJ 1/14; H01J 19/06 posed outer surfacef the fi t f the two zones is [58] Field of Search 313/311 2I83 sitionedin the direction of the arc discharge during 29/25'14' 25-17 operationof the lamp and is closer proximity thereto I during operation than theouter surface of the second [56] References Cited zone. The first zoneconsists of a refractory material I UNlTED STATES PATENTS which has beensintered at a temperature higher than 2,492,142 12/1949 Gcrmeshausen313/346 R the temperature of the arc discharge to form a 2 83(),2184/1958 Bcggs 313/346 R tered interconnected porous material. The second2,975,322 3/1961 Cockrill 313/346 R zone consists of a sintered porousmixture of refrac- 3,()l0,046 ll/l96l Dailcy et al 313/346 R torymaterial and emissive materiaL 3,076,915 2/1963 Gal et al. 313/346 R 23,244,929 4/1966 Kuhl 313/346 R X 21 Claims, 2 Drawin Flgul'es US.Patent Ot. 7,1975 3,911,309

- FIG. I FIG. 2

ELECTRODE COMPRISING A POROUS SINTERED BODY Electrodes having a sinteredbody are known. British Pat. No. 943,535 discloses a porous cathode bodycomposed of tungsten which is saturated with an emissive substance. Thecathode body also contains a second emissive substance having a lowerelectron work function disposed in recesses of the surface facing thearc discharge. This second emissive substance is used for initiation ofthe arc discharge. US. Pat. No. 3,244,929 discloses a sintered electrodebody composed of several zones formed from different emissive mixtures.These mixtures are compressed and sintered about an electrode rod. Thezones of higher electron work function and heat resistance are providedin the direction of the space which contains the discharge arc duringoperation. The zones having a, lower electron work function arepositioned away from the space adapted to be occupied by the arcdischarge during operation, i.e., the zones of lower work function areat the other side of the electrode.

The emissive characteristic of an electrode which are most satisfactoryduring operation of the arc discharge are different from the emissivecharacteristics which are most satisfactory for initiating said aredischarge. The aforedescribed prior art constructions provide electrodeshaving at least two surfaaces having different emissive characteristics,one more suitable for initiation of the arc, and the other more suitablefor operation of the arc. The present invention provides an improvedelectrode for discharge lamps having superior characteristics forinitiation of the arc and operation of the are.

SUBJECT MATTER OF THE INVENTION The present invention provides asintered electrode for use in gaseous and/or vapor discharge lamps and,particularly, high-pressure lamps. The sintered electrode contains acentral electrode rod having one portion substantially enclosed by twocontiguous porous sintered zones each of which has an exposed outersurface. The exposed outer surface of the first zone is positioned inthe direction of the electric arc discharge during operation of the lampand in closer proximity thereto during operation than the outer surfaceof the second zone. The first zone consists of a sintered interconnectedporous refractory material which has been sintered at a highertemperature than the temperature reached during service of the dischargelamp, i.e., at a higher temperature than the temperature of the aredischarge proximite the electrode surface. The second zone consists of asintered mixture of refractory material and emissive material.

Both zones may be formed by sintering high temperature refractorymetals, e.g., tungsten, or high temperature refractory metalliccompounds, e.g., zirconium carbide, lanthanum boride, etc.

The first zone which does not contain emissive material duringpreparation of the electrode has a density of between 8 and 16 grams percubic centimeter when using tungsten as the refractory material. Thepreferred density is from 10 to 14 grams per cubic centimeter. Thesecond sintered zone contains an emissive substance with the refractorymaterial. The relative amounts, for example of a refractory materialsuch as tungsten, and the emissive material vary from about 30 .topercent by weight of refractory material to about 70 to 30 percent byweight of the emissive material. The percent by weight is based on thetotal. The preferred ratio is 50-+50 weight percent of each.

The emissive material or substance includes those known to the prior artto be suitable for this purpose, such as the oxides of barium, calciumand thorium, and

also'barium cerate.

The first zone has a sintered structure of interconnected pores. Thesecond 'zone which is contiguous with the first zone has a refractorymetal skeleton and contains emissive material in interconnected pores.Because of the contiguous relationship of the two zones, the emissivematerial from the second zone flows into the outer first zone and to thesurface thereof during operation. This prevents sputtering of therefractory material at the position or positions at which the arcdischarge appends during operation of the lamp: The terms appends andappended refer to the position at which the arc discharge makes contactwith the sur face of the electrode during operation of the lamp andalsothe position at which contact is made during ig nition of the lamp.As noted, the arc apperidsto the second zone during initiation of thedischarge and occurs immediately when the arc strikes or forms. The areappends to the outer surface of the first zone during operation of thelamp. In a preferred embodiment of the present invention, the secondzone is positioned within a cavity of the first zone which is shaped asa cap over the second zone. The outer surface of the firstzone, i.e.,the cap, may have such shapes. as a cylindrical. surface, a conicalsurface or a frustoconical shape. ,The opening of. the cap in which thesecond zone material is encompassed faces away from the direction of thearc discharge during operation of the lamp.

The electrode rod which is preferably centrally positioned in thesintered electrode either protrudes with its end extending through thecap-shaped first zone in the direction of the arc discharge duringoperation, or it may terminate with the cap-shaped zone. This preferredembodiment provides a second zone for initiation of the are having anexposed surface of large area extending in the direction away from thearc discharge during operation.

The invention will be described by way of example I with reference tothe accompanying drawings wherein:

FIG. 1 is a section through one preferred embodiment of the electrode;and

FIG. 2 is a section through another preferred embodiment of theelectrode.

In FIG. 1 the electrode rod 1 of thoriated tungsten having a diameter of1.2 mm projects with its tip 2 approximately 1.5 mm out of the sinteredbody 3. The sintered body 3 is of cylindrical shape. Its length of about5 mm is slightly larger than its diameter. It comprises a first zone 4and a second zone 5. The first zone 4 has the shape of a cylindrical capwith bell-shaped cavity 6. It is made of finely sintered porous tungstenwith a density of 12 g/cc. The cavity is almost completely filled with asintered emissive mixture of 50 wt. percent of tungsten and 50 wt.percent of a mixture of oxides of barium, calcium, and thorium whichforms the second zone 5. The outer surface 7 of the first zone 4 withinthe lamp is located more closely to the discharge than the outer surface8 of the second zone 5.

In FIG. 2 the first zone 4 has the shape of a conical cap 9. Theelectrode rod 1 terminates with its tip 2 within the surface boundary ofcap 9.

The sintered electrodes of the present invention are advantageousprepapred using powder metallurgic techniques. The first electrode zonehas particularly superior mechanical and high temperature properties.The sintered material having interconnected pores is suitably preparedby admixing the refractory material, e.g., tungsten, or zirconiumcarbide, with a powdered material which will not alloy with therefractory material and which has a sufficiently low boiling point sothat it will vaporize during sintering. Suitable low temperaturematerials include zinc, copper, gold, etc. When using tungsten as therefractory material, zinc is particularly suited as the low boilingpoint material because it additionally functions as a pressing aidduring the pressing of the shaped body.

The mixture of the refractory material and the low boiling pointmaterial is pressed to the desired shape using conventional powdermetallurgical techniques. The electrode rod may be pressed into thesintered body at the same time. The slug is sintered at a temperatureabove the vaporization temperature of the low boiling point material.During this first sintering step, the low boiling material evaporatesleaving a sintered, porous, skeletal structure which is then subjectedto a higher temperature sintering, with temperatures up to 2600C. Thehigh degree of porosity and the interconnected porous structure ismaintained during such high temperature sintering. Scan electronmicroscopic photographs of electrodes so prepared disclose that thebridges between the individual grains forming the slug, attain a widthduring high temperature sintering almost sufficient to match the graindimensions This accounts for the high temperature strength. Suchstructure is attained while maintaining the interconnected porestructure.

The second zone is prepared utilizing a powder mixture of the refractorymaterial, e.g., a metal such as tungsten, and the emissive material.This powder mixture may be attached to the first zone. When the firstzone is in the form of a hollowshaped body, the powder mixture of thesecond zone material may be introduced into the cavity of the first zoneand the two sintered together at a lower temperature of up to about2000C. This forms the emitter-containing second zone. During thepreparation of the first zone, the sintering may be in two sequentialsteps as aforedescribed, or in a single 'sintering wherein theevaporation of the low boiling point material and the final sinteringare achieved in a single continuous sintering operation.

When utilizing certain refractory materials, such as tungsten, an inertatmosphere or vacuum is required to prevent oxidation, as is well knownin the art.

The advantage of sintered electrodes when compared with wire coilelectrodes is that the sintered electrodes are cheaper and more simpleto manufacture. However, their stability at high electrical and thermalloads has been insufficient to attain a warranted lifetime of more than5000 hours. Accordingly, they were not used in lamps with a wattageinput exceeding 125 watts.

The electrodes of the present invention have superior servicecharacteristics and are suitable for use in discharge lamps with awattage input of 400 watts and even higher. This is possible because thehigh sintering temperature not only improves the mechanical propertiesand high temperature stability of the porous structure, but alsoimproves the electrical conductivity thereof. As noted, the hightemperature at which the first zone of the electrode is sintered resultsin a structure having high temperature stability without sinteringresulting in closing of the pores even with a high load. As aconsequence, the supply of successive doses of emissive material to thesurface of the first zone from the second zone is assured duringoperation.

Due to the bell shape of the preferred embodiment of the presentinvention and the consequent large area provided at which the arcdischarge may append during start-up of the lamp, and the positioning ofthis large area at the rear of the electrode away from the are dischargearea during operation, there results a favorable rapid heating of thespace behind the electrode as a consequence of the high currentintensity during startup and provides a rapid transition of the arcdischarge to the lower current density station high-pressure phase. Thelow heat capacity of the electrode produced in accordance with thepresent invention contributes to this effect. The bell-shaped cavity ofthe preferred embodiments results in effective protection of theemitterrich second zone during stationary operation of the lamp.

When using an electrode with tungsten as the refractory material in alamp in which the temperature immediately at the position where the arcdischarge touches the electrode is, for example, about 2000C, thesintering temperature of the first zone preferably ranges be tween 2400and 2600C. The sintering temperature of the second zone which includesemissive material of preferably barium oxide 19.5 percent by wt.,calcium oxide 5.5 and thorium oxide 75.0 is between l600 and 2()OOC.

We claim:

1. A sintered electrode for discharge lamps adapted to operate with anelectric discharge are appended to an outer surface thereof, comprisinga central electrode rod having one portion substantially enclosed byfirst and second contiguous porous sintered zones each of which has anexposed surface, said exposed surface of the said first zone beingpositioned in the direction of the arc discharge during operation of thelamp and in closer proximity thereto during operation than said exposedsurface of said second zone, said first zone consisting ofinterconnected porous material refractory sintered at a temperaturebetween 2400C and 2600C, and said second zone consisting of a sinteredporous mixture of refractory material and emissive material.

2. The sintered electrode of claim 1 wherein said refractory material ofsaid first and said second zones is a high temperature refractory metal.

3. The sintered electrode of claim 2 wherein said high temperaturerefractory metal is tungsten.

4. The sintered electrode of claim 3 wherein said first zone has adensity of from 8 to 16 g/cc.

5. The sintered electrode of claim 4 wherein said second zone consistsof tungsten and an emissive material in relative amounts of from 30-70percent by weight of tungsten to -30 percent of said emissive material.

6. The sintered electrode of claim 5 wherein said first zone has adensity of from 10-14 g/cc.

7. The sintered electrode of claim 6 wherein said second zone containstungsten and emissive material in an amount of about 50 percent of each.

8. The sintered electrode of claim 7 wherein said emissive material isat least one compound selected from the group consisting of bariumoxide, calcium oxide, thorium oxide, and barium cerate.

9. The sintered electrode of claim 8 wherein said first zone is formedin the shape of a bell-shaped cap with said electrode rod protrudingfrom the open end of said cap and with said second zone positionedwithin the cavity of said cap.

10. The sintered electrode of claim 9 wherein said electrode rod alsoprotrudes through the other end of said cap-shaped first zone.

11. The sintered electrode of claim 1 wherein said high temperaturerefractory metal is tungsten.

12. The sintered electrode-of claim 1 wherein said emissive material isat least one compound selected from the group consisting of bariumoxide, calcium oxide, thorium oxide, and barium cerate.

13. The sintered electrode of claim 1 wherein said second zone consistsof tungsten and an emissive material in relative amounts of from 30-70percent by weight of tungsten to 70-30 percent of said emissivematerial.

14. The sintered electrode of claim 1 wherein said first zone is formedin the shape of a bell-shaped cap with said electrode rod protrudingfrom the open end of said cap and with said second zone positionedwithin the cavity of said cap.

15. The sintered electrode of claim 1 wherein said electrode rod alsoprotrudes through the other end of said cap-shaped first zone.

16. A method of manufacturing a sintered electrode for discharge lampsadapted to operate with an electric discharge are appended to an outersurface thereof having a central electrode rod with one portionsubstantially enclosed by first and second contiguous porous sinteredzones each of which has an exposed surface, said exposed surface of thesaid first zone being positioned in the direction of the arc dischargeduring operation of the lamp and in closer proximity thereto duringoperation than said exposed surface of said second zone; comprisingadmixing a powdered refractory material with a low boiling pointmaterial which does not alloy with said refractory material and whichvaporizes at the sintering temperature of said refractory material;compressing said admixture in a shaped die around said electrode rod toform a pressed compact surrounding said electrode rod; sintering saidpressed compact and electrode rod at a temperature above thevaporization temperature of said low boiling point material whereby saidlow boiling point material completely vaporizes; and then sintering at ahigher temperature up to about 2600C to obtain an interconnected porousfirst zone substantially enclosing a portion of said central electroderod having high mechanical and high temperature properties during hightemperature service; and forming a shaped admixture of refractorymaterial and said emissive material in the cavity of said bell-shapedsintered refractory material and sintering at a lower temperature up toabout 2000C.

17. The method of claim 16 wherein said electrode rod is thoriatedtungsten, said refractory material in said first zone is selected fromthe group consisting of tungsten, zirconium carbide and lanthanumboride, and said low boiling point material is zinc.

18. The process of claim 17 wherein said refractory material is tungstenand wherein said tungsten is sintered at a temperature between 2400C and2600C to obtain said interconnected porous first zone.

19. The process of claim 18 wherein said emissive material consists ofbarium oxide, calcium oxide and thorium oxide, and wherein the finalsintering is at a temperature between 1600C and 2000C.

20. The sintered electrode of claim 7 wherein said emissive materialconsists of 19.5 percent by weight of barium oxide, 5.5 percent ofcalcium oxide and percent of thorium oxide.

21. The sintered electrode of claim 11 wherein said second zonecomprises 50 percent tungsten and 50 percent emissive material and saidemissive material consists of 19.5 percent by weight of barium oxide,5.5 percent of calcium oxide and 75 percent of thorium oxide.

1. A SINTERED ELECTRODE FOR DISCHARGE LAMPS ADAPTED TO OPERATE WITH ANELECTRIC DISCHARGE ARC APPENDED TO AN OUTER SURFACE THEREOF, COMPRISINGA CENTRAL ELECTRODE ROD HAVING ONE PORTION SUBSTANTIALLY ENCLOSED BYFIRST AND SECOND CONTIGUOUS POROUS SINTERED ZONES EACH OF WHICH HAS ANEXPOSED SURFACE, SAID EXPOSED SURFACE OF THE SAID FIRST ZONE BEINGPOSITIONED IN THE DIRECTION OF THE ARC DISCHARGING DURING OPERATION OFTHE LAMP AND IN CLOSER PROXIMITY THERETO DURING OPERATION THAN SAIDEXPOSED SURFACE OF SAID SECOND ZONE, SAID FIRST ZONE CONSISTING OFINTERCONNECTED POROUS MATERIAL REFRACTORY SINTERED AT A TEMPERATUREBETWEEN 2400*C AND 2600*C, AND SAID SECOND ZONE CONSISTING OF A SINTEREDPOROUS MIXTURE OF REFRACTORY MATERIAL AND EMISSIVE MATERIAL.
 2. Thesintered electrode of claim 1 wherein said refractory material of saidfirst and said second zones is a high temperature refractory metal. 3.The sintered electrode of claim 2 wherein said high temperaturerefractory metal is tungsten.
 4. The sintered electrode of claim 3wherein said first zone has a density of from 8 to 16 g/cc.
 5. Thesintered electrode of claim 4 wherein said second zone consists oftungsten and an emissive material in relative amounts of from 30-70percent by weight of tungsten to 70-30 percent of said emissivematerial.
 6. The sintered electrode of claim 5 wherein said first zonehas a density of from 10-14 g/cc.
 7. The sintered electrode of claim 6wherein said second zone contains tungsten and emissive material in anamount of about 50 percent of each.
 8. The sintered electrode of claim 7wherein said emissive material is at least one compound selected fromthe group consisting of barium oxide, calcium oxide, thorium oxide, andbarium cerate.
 9. The sintered electrode of claim 8 wherein said firstzone is formed in the shape of a bell-shaped cAp with said electrode rodprotruding from the open end of said cap and with said second zonepositioned within the cavity of said cap.
 10. The sintered electrode ofclaim 9 wherein said electrode rod also protrudes through the other endof said cap-shaped first zone.
 11. The sintered electrode of claim 1wherein said high temperature refractory metal is tungsten.
 12. Thesintered electrode of claim 1 wherein said emissive material is at leastone compound selected from the group consisting of barium oxide, calciumoxide, thorium oxide, and barium cerate.
 13. The sintered electrode ofclaim 1 wherein said second zone consists of tungsten and an emissivematerial in relative amounts of from 30-70 percent by weight of tungstento 70-30 percent of said emissive material.
 14. The sintered electrodeof claim 1 wherein said first zone is formed in the shape of abell-shaped cap with said electrode rod protruding from the open end ofsaid cap and with said second zone positioned within the cavity of saidcap.
 15. The sintered electrode of claim 1 wherein said electrode rodalso protrudes through the other end of said cap-shaped first zone. 16.A method of manufacturing a sintered electrode for discharge lampsadapted to operate with an electric discharge arc appended to an outersurface thereof having a central electrode rod with one portionsubstantially enclosed by first and second contiguous porous sinteredzones each of which has an exposed surface, said exposed surface of thesaid first zone being positioned in the direction of the arc dischargeduring operation of the lamp and in closer proximity thereto duringoperation than said exposed surface of said second zone; comprisingadmixing a powdered refractory material with a low boiling pointmaterial which does not alloy with said refractory material and whichvaporizes at the sintering temperature of said refractory material;compressing said admixture in a shaped die around said electrode rod toform a pressed compact surrounding said electrode rod; sintering saidpressed compact and electrode rod at a temperature above thevaporization temperature of said low boiling point material whereby saidlow boiling point material completely vaporizes; and then sintering at ahigher temperature up to about 2600*C to obtain an interconnected porousfirst zone substantially enclosing a portion of said central electroderod having high mechanical and high temperature properties during hightemperature service; and forming a shaped admixture of refractorymaterial and said emissive material in the cavity of said bell-shapedsintered refractory material and sintering at a lower temperature up toabout 2000*C.
 17. The method of claim 16 wherein said electrode rod isthoriated tungsten, said refractory material in said first zone isselected from the group consisting of tungsten, zirconium carbide andlanthanum boride, and said low boiling point material is zinc.
 18. Theprocess of claim 17 wherein said refractory material is tungsten andwherein said tungsten is sintered at a temperature between 2400* C and2600* C to obtain said interconnected porous first zone.
 19. The processof claim 18 wherein said emissive material consists of barium oxide,calcium oxide and thorium oxide, and wherein the final sintering is at atemperature between 1600* C and 2000*C.
 20. The sintered electrode ofclaim 7 wherein said emissive material consists of 19.5 percent byweight of barium oxide, 5.5 percent of calcium oxide and 75 percent ofthorium oxide.
 21. The sintered electrode of claim 11 wherein saidsecond zone comprises 50 percent tungsten and 50 percent emissivematerial and said emissive material consists of 19.5 percent by weightof barium oxide, 5.5 percent of calcium oxide and 75 percent of thoriumoxide.